Gas turbine engines typically include a fan delivering air into a compressor. The air is compressed in the compressor and delivered into a combustion section where it is mixed with fuel and ignited. Products of this combustion pass downstream over turbine blades, driving them to rotate. Turbine rotors, in turn, drive the compressor and fan rotors. The efficiency of the engine is impacted by ensuring that the products of combustion pass in as high a percentage as possible across the turbine blades. Leakage around the blades reduces efficiency. Thus, a blade outer air seal (BOAS) is provided radially outward of the blades to prevent leakage.
The BOAS is spaced from a radially outer part of the blade by a tip clearance. The BOAS is traditionally associated with a carrier element that is mounted to an engine case. Since the blades, the BOAS, and the structure that support the BOAS are different sizes and/or are formed of different materials, they respond to temperature changes in different manners. As these structures expand at different rates in response to temperature changes, the tip clearance may be reduced and the blade may rub on the BOAS, or the tip clearance may increase reducing efficiency, both of which are undesirable.
Clearance control systems are used to control the tip clearance under different operational conditions. Traditional clearance control systems utilize valves and manifolds to direct fan air to specific engine case locations. The cooling air thermally shrinks the engine case at these locations to improve tip clearance and thus fuel burn. However, these manifolds and valves are large, heavy, and expensive. These systems can also be slow to respond and provide limited clearance improvement. By further reducing tip clearances increasing engine efficiency demands can be met.